Fluid-cooled traveling wave tube



Mamh 1967 E. L. LIEN 3,309,556

FLUID-COOLED TRAVELING WAVE TUBE Filed Sept. 11, 1964 2 Sheets-Sheet l COLING FLUID COOLING i 11 FLUID L? T INVENTOR Erling L. Lien ATTORNEY 14, 1967 E, L LIEN I FLUID-COOLED TRAVELING WAVE TUBE 2 Sheets-Sheet 2 Filed Sept. 11, 1964 FIG. 7.

FIG. 6.

} FIG. 9.

' FIG. 8.

INVENTOR United States Patent Ofiice 3,309,556 Patented Mar. 14, 1967 3,309,556 FLUID-(300L111) TRAVELING WAVE TUBE Erling L. Lien, Mountain View, Calif, assignor to Westinghouse Eiectric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Sept. 11, 1964, Ser. No. 395,651 7 8 Claims. (Cl. 3153.5)

The present invention relates to microwave tubes and, more particularly, to a tube which contains a slow wave propagating helix that is cooled by a fluid.

In high power traveling wave tubes which have a slow wave propagating structure such as a helix that is cooled by a fluid it is necessary to electric-ally isolate the fluid supply and return connections from the high frequency electromagnetic fields in the tube. At frequencies above 1000 megacycles/second, this problem has been solved by using heavily loaded reentrant resonant cavities as coupling units for transferring electromagnetic energy between the helix and transmission line coupling connections positioned externally of the vacuum envelope for the tube. However, at frequencies under 1000 megacycles/second the geometrical size of these cavities become too large to be practical.

Another arrangement for electrically isolating the cooling fluid supply and return connections is to employ a quarter wavelength coaxial T-junction having a shorted end and to extend the helix out through the shorted end. A cooling fluid connection for the helix is made at this shorted end. One example of such a construction is shown and described in US. Patent No. 2,833,955, granted May 6, 1958. A disadvantage of this construction is that the T-junction is impractically large at low frequencies. At 375 mega-cycles per second, for example, the length of the T-junction must be approximately eight inches. Moreover, the useful frequency bandwidth of the T-junction is relatively narrow.

It is a primary object of the present invention, therefore, to provide a compact construction for isolating cooling fluid supply and return connections from the high frequency electromagnetic fields in a microwave structure that is cooled by a cooling fluid.

Another object is to provide a means, as is described in the preceding paragraph, which has a relatively wide useful frequency bandwidth.

A further object is to provide a microwave tube that has an electromagnetic wave energy propagating structure and improved means for supplying a cooling fluid to the propagating structure without disturbing the microwave fields in the tube.

Still another object is to provide a high power traveling wave tube that has a slow wave propagating structure which is cooled by a fluid, there being compact constructions at both the input end and the output end of the tube (1) for transferring electromagnetic energy between the slow wave propagating structure and input and output transmission line coupling connections positioned externally of thevacuum envelope for the tube and (2) for transferring a cooling fluid to and from the propagating structure without disturbing the electromagnetic fields carried by the microwaveparts of the tube.

Yet another object is to provide a traveling wave tube, as is described in the preceding paragraph, which has a relatively wide useful frequency bandwidth.

The foregoing objects as well as other objects and advantages of the invention are achieved by a microwave tube that comprises a vacuum envelope, a slow wave propagating structure supported within said envelope, and a transmission line section supported within the envelope at each end of the slow wave propagating structure for transferring electromagnetic energy between the propagating structure anda transmission line coupling connection positioned externally of the envelope. The slow wave propagating structure is adapted to be cooled by a fluid that is passed through each of the transmission line sections at the ends of the propagating structure. Cooling fluid is transferred between each transmission line section and a cooling fluid connection positioned externally of the vacuum envelope by a helical section of tubular conductor. A shorting plane is positioned at the end of each helical section to form a resonant section that presents a high shunt impedance to the transmission line section over a wide frequency band.

The details of the present invention will become more apparent from the following description of the accompanying drawing, wherein:

FIGURE 1 is a schematic illustration in longitudinal section of a traveling wave tube that is provided with input and output constructions for transferring both electromagnetic energy and a cooling fluid to and from a slow wave propagating helix in accordance with a first embodiment of the invention;

FIG. 2 is a cross-sectional view taken from the line II-II in FIG. 1;

FIG. 3 is a sectional view taken from line III-III in FIG. 2;

FIG. 4 is a sectional view taken from line IV-IV in FIG. 2;

FIG. 5 is a sectional view similar to the view in FIG. 4 but showing an alternative embodiment of the invention;

FIG. 6 is a schematic illustration that identifies the electrical parts of the construction for transferring both electromagnetic energy and a cooling fluid to or from a traveling wave tube helix;

FIG. 7 is a schematic view showing part of the helix and the shield shown in FIG. 6;

FIG. 8 is a schematic illustration of a diiferent embodiment of a construction for transferring high frequency electromagnetic energy and a cooling fluid to or from a traveling wave tube helix; and

FIG. 9 is a schematic illustration similar to the view in FIG. 8 of still another embodiment of the invention.

Referring to the drawing, wherein corresponding primed and un'primed reference numerals designate similar parts, there is illustrated in FIGS. l-4 a traveling wave tube comprising a slow wave propagating structure in the form of a conductive helix 11, an electron gun 13 positioned at one end of the helix for producing and directing an electron beam through the helix, and a collector electrode 15 positioned at the other end of the helix for receiving the electrons of the beam. The helix 11 is made from highly conductive material and is tubular for carrying a cooling fluid for dissipating heat.

A conventional magnetic focusing system, not shown, is employed to ensure that the beam retains a constant diameter along the helix 11. Both the electron gun 13 and the collector electrode 15 are conventional. The tube is designed for highpower operation over a wide frequency band below 1000 megacycles per second at a center frequency of 375 megacycles per second, for example.

At the electron gun end of the helix 11 a construction 17' isused for transferring electromagnetic energy from an input coaxial line coupling connection, not shown, to the helix and also for transferring a cooling fluid to the helix from a cooling fluid connection 27' that is connected by pipe 18' to a cooling fluid recirculating system, not shown. At the electron collector end of the helix, a similar construction 17 is used for transferring electromagnetic energy to an output coaxial line coupling c0nnection 19, shown in FIG. 2, and also for transferring the cooling fluid from the helix to a cooling fluid connnection 27 that is connected by pipe 18 to the cooling fluid recirculating system. The coaxial line coupling connections and the cooling fluid connections 18' and 18 are posi- 3 tioned externally of the vacuum envelope for the travel= ing Wave tube. The constructions 17 a nd 17' contain similar parts. Therefore, the details of the construction 17 only will be described. p

The construction 17 comprises a hollow cylindrical outer conductor 20, a hollow cylindrical inner conductor 21 that is supported coaxially Within conductor 20, and a helical section 23 that is supported coaxially about the inner conductor 21 for transferring cooling fluid from the helix 11 to the fiuid connection 27. The helical section 23 is made from a tubular piece of highly conductive material.

One end of the helical section 23 is shorted upon an end Wall 25 of the conductor 20. Wall 25 constitutes a shorting plane and is at a radio frequency ground. This shorting plane is substantially perpendicular to the axis of helical section 23. The cooling fluid connection 27 is mounted upon the wall 25.

The other end of the helical section 23 is coupled to an end of the helix 11 by a connector 29, shown in FIG. 2. Connector 29 passes coaxially through a tubular opening 31 in the cylindrical wall of conductor 21. The connector 29 is made from a tubular piece of highly conductive material.

Included as part of the construction 17 is an electromagnetic energy transfer means comprising a strip transmission line section consisting of conductive member 33, which has a rectangular cross-section, and a ground plane conductor 35, shown in FIGS. 2 and 4. One end of member 33 is connected to the end of the inner conductor of the output coaxial line coupling connection 19. The other end of member 33 is electrically connected to the slow wave propagating helix by the connector 29. The connector 29 and one end of the helical section 23 pass through an L-shaped passage in member 33 as is illustrated in FIG. 2.

The ground plane conductor 35 is supported perpendicularly between the end walls of the hollow cylindrical outer conductor 20 in proximity with the member 33. The conductors 33 and 35 form a strip transmission line section which has a characteristic impedance that has a value which equals the value of the characteristic impedance K for the coaxial line coupling connection 19.

An inner surface 37 of the conductor 21 is shaped so that it is conical, as is shown in FIG. 1. The surface 37 constitutes a shield which extends along helix 11 for several wavelengths to give a gradual transition from the helix circuit impedance to a line-above-ground plane impedance K having a value that equals the value of the strip transmission line impedance K. The portion of the connector 29 and the surrounding conductive surface which defines the tubular opening 31 form a short section of coaxial line for connecting the shielded end of helix 11 to the strip transmission line section that is formed by conductors 33 and 35. The characteristic impedance of this coaxial line section equals that of this strip transmission line section. g

The helical section 23 constitutes a shunt connection to the strip transmission line. At the center frequency of operation of the traveling wave tube, the length of the helical section 23 from the junction with the strip transmission line to the shorting plane 25 is kg/4 where Ag is the propagation wavelength of the electromagnetic wave along the helical section in an axial direction. The total length of the helical conductor measured around the helical section is approximately one quarter free-space wavelength.

A very important feature concerning the helical section 23 is that it presents a high shunt impedance to the strip transmission line section over a large frequency band. Therefore, the helical section 23 has a minimum adverse electrical affect upon the strip transmission line section thereby minimizing reflections of electromagnetic energy in the transmission line system. The distance from the helical section 23 to the inner cylindrical wall of the 4 conductor 20, although not critical, should not be so small that it reduces the impedance of the helical section 23. As a practical matter, however, this wall has to be quite close to section 23 to reduce the impedance noticeably.

The helical section 23 represents, when it has a length of 7\g/4, an infinite shunt impedance to the strip transmission line (disregarding losses). The value of the shunt impedance at frequencies off resonance depends upon the characteristic circuit impedance of the helical section 23 and the degree of coupling between section 23 and the strip transmission line formed by conductors 33 and 35. The characteristic circuit impedance of the helical section 23 is relatively large compared with the im pedance of a standard fifty ohm coaxial line, for example. A low degree of coupling is obtained by connecting the helical section to the low electric field region of the strip transmission line, i.e. to the side of conductor 33 that is farthest from the ground plane conductor 35. The relatively large circuit impedance of the helical section and the low degree of coupling with the strip transmission line section serve to ensure that the value of the shunt impedance presented by helical section 23 to the strip transmission line is kept high at frequencies oif resonance over a wide frequency band. This is an important feature of the invention.

The vacuum envelope for the traveling wave tube is formed by a hollow tubular conductor 39, by the conductors 20 and 20, and by conductive housings 41 and 43 which contain the electron gun and electron collector, respectively. Both the traveling wave helix 11 and the helical sections 23 and 23' are supported coaxially within the vacuum envelope. Each of the coaxial line coupling connections at the input end and the output end of the tube contains a dielectric bead such as 45, shown in FIG. 2, for preserving the vacuum in the tube.

The operation of the traveling wave tube is conventional. Electromagnetic energy'from a suitable source of microwave energy, not shown, is supplied to the input coaxial line coupling connection, not shown, which is mounted externally of construction 17 upon the conductor 20. Within the construction 17 the strip transmission line conductors 33 and 35 and the coaxial line extension, which includes tubular opening 31', transfers the microwave energy efficiently, with minimum reflection, to the slow wave propagating helix 11. Interaction between the electron beam and the microwave energy traveling along helix 11 produces amplification of the microwave energy. Within the construction 17 at the output end of the tube the coaxial line extension formed by conductors 29 and 31 and the strip transmission line section formed by conductors 33 and 35 transfer the amplified microwave energy efficiently with minimum reflection, to the coaxial line coupling connection 19 which is coupled to a suitable microwave load, not shown.

The helix 11 is kept cool by the cooling fluid that is circulated therethrough. Both of the cooling fluid connections 27' and 27 are effectively at a radio frequency ground and have no effect upon the electromagnetic fields within the tube. The helical sections 23' and 23 for transferring cooling fluid to and from the helix 11 have a minimum adverse effect upon the electromagnetic fields over a wide frequency band for reasons that have been described.

A schematic illustration of the electrical parts of the construction 17 or 17 is given in FIGS. 6 and 7. It is seen that the means for transferring electromagnetic energy from a coaxial line coupling connection having an impedance K to the traveling wavetube helix beyond the end of its impedance matching conical shield comprises a strip line section and a short section of coaxial line having equal characteristic impedances K. In a typical arrangement, the impedance K is 50 ohms. The helical section for transferring a cooling fluid to the traveling wave tube helix is represented by numeral 45.

The shorting plane positioned at the end of the helical section 45 is indicated by numeral 46.

An alternative embodiment of the construction 17 is illustrated in FIG. 5. This embodiment comprises a hollow cylindrical outer conductor 50, a hollow cylindrical inner conductor 21 that is supported coaxially within conductor 50, and a helical section 53 made from a tubular conductor that is supported coaxially about the inner conductor 21 for transferring cooling fluid from a traveling wave tube helix, not shown, to the cooling fluid connection 27". In this embodiment, the helical section 53 has several turns. One end of section 53 is shorted upon an end wall 55 of the conductor '50. Wall 55 constitutes a shorting plane that is spaced from the connection of section 53 to conductor 33 by Ag/4. Since the helical section 53 has several turns the volume of the present embodiment of construction 17 is smaller than in the previously described embodiment shown in FIGS. 1 to 4, assuming the same frequency of operation.

In the construction shown in FIG. 5, the end wall 57 of conductor 50 is the ground plane of a strip transmission line section that is formed by conductor 33" and wall 57. Thus, compared with the embodiment of the invention that is illustrated in FIGS. 1-4, a separate strip transmission line ground plane is not required. In FIG. 5 the parts referred to by double-primed reference numerals are similar to the parts referred to by the corresponding reference numerals in the embodiment of construction 17 shown in FIGS. 1-4.

Yet another embodiment of the construction 17 is illustrated in FIG. 8. This other embodiment comprises a helical section 61 that is supported within a hollow cylindrical conductor 63 having a pair of end walls 65 and 67. The conductor 63 and end walls 65 and 67 are part of a vacuum envelope. The end walls 65 and 67 form shorting planes which are spaced by tg/2 to make helical section'61 resonant at the center frequency of operation.

Mounted coaxially within the helical section 61 is a tubular conductor 69 having a conically shaped inner surface. Conductor .69 shields an end portion of a traveling wave tube helix 71 for several wavelengths and performs an impedance matching function. The helical section 61 should be spaced by a minimum distance from conductor 69 so that the impedance of the helical section is not appreciably diminished.

Electromagnetic energy is transferred between a coaxial line coupling connection 73, positioned externally of conductor 63, and traveling wave tube helix 71. This transfer is accomplished by a section of transmission line positioned within conductor 63. This transmission line section comprises an extension 75 of the inner conductor of connection 73, part of one of the turns of the helical section 61, and a connector 77 that passes coaxially through a hole 81 that extends through conductor 69 to form a short section of coaxial line. The size of the hole 81 through which the connector 77 passes is selected to give a coaxial line impedance of 50 ohms, for example, to equal the impedance of the coaxial line coupling connection 73. The connector 77 and the helical section 61 are tubular so that a' cooling fluid can be transferred between an externally mounted cooling fluid connection 79 and the helix 71.

The transfer of electromagnetic energy between the external coaxial line coupling connection 73 and the traveling wave tube helix 71 depends upon the axial lo- 7 cation of the connection of connector extension to the helical section 61 and the location of the connectical. The smallest axial extension is equivalent to hg/Z at the midband operating frequency (where Ag is the helix guide wavelength in the axial direction of the helical conductor section 61). The axial extension of helical conductor section 61 for operation at 375 megacycles per second and with a chosen phase velocity equal to one-tenth times the velocity of light, for example, is approximately 1.5 inches. In this example, the maximum radial dimension of the hollow cylindrical outer conductors 63 is in the vicinity of 5-6 inches so as not to reduce the relatively high impedance of the helical section compared with a 50-ohm coaxial line impedance.

Still another embodiment of the present invention is shown in FIG..9. In this embodiment a helical conductor section 91 is supported coaxially within a hollow cylindrical outer conductor 83. The helical conductor section 91 is formed from a length of strip-like conductor. The opposite ends of the helical conductor section 91 are connected to opposite ends of the outer conductor 83 by short-circuiting planes formed by conductive end walls 85 and 87 to form a resonant section.

An end portion of a traveling wave tube helix 89 extends coaxially into the helical section 91. One end of the helix 89 is connected to the section 91 by a radially extending conductive connector 92 that passes through one of the turns of the helical conductor section 91 at a first point along section 91. A coaxial transmission line 93 is coupled tothe resonant line by an extension 95 that is connected to a second point along the helical conductor section 91. The points along the helical conductor section 91 to which the connector 92 and the extension 95 are connected are adjusted to achieve optimum broad band transmission characteristics.

The traveling wave tube helix 89 is made from a tubular conductor to permit a cooling fluid to be passed through the helix. The connector 92 also is formed from a tubular conductor that is connected between one end of the helix 89 and one end of a helical conductor section 97. The section 97, which is made from a tubular conductor, encircles the end turns of the helical conductor section 91. The conductor 97 passes through the end wall 85 and is connected to a cooling fluid pipe connection 99.

The principal difference between the constructions shown in FIGS. 8 and 9 is in the means for coupling the traveling wave tube helix to the externally mounted coaxial line coupling connector. In FIG. 9, the electromagnetic energy is transferred partly by the common linkage of the electric and magnetic field between the helical section 91 and the traveling wave helix 89 and partly by direct coupling by connector 92. The first type of coupling is analagous to the one for conventional coupled helices used in traveling wave tubes.

At a predetermined operating frequency the radial dimension of the structure shown in FIG. 9 can be made smaller than the radial dimension of the structure shown in FIG. 8 since the resonant helical section 91 can be quite close to the traveling wave tube helix 89. In the FIG. 9 construction there is no inner conical shield, such as 69 in FIG. 8, from which the resonant helix must be spaced by a minimum distance.

What is claimed is:

1. A microwave tube comprising a vacuum envelope, an electromagnetic wave energy propagating structure positioned within said envelope, a transmission'line coupling connection positioned externally of said envelope, an electromagnetic energy transmission line section positioned within said envelope for transferring electromagnetic energy between said coupling connection and said propagating structure, a cooling fluid connection positioned externally of said envelope, a helical section positioned within said envelope for transferring a cooling fluid between said cooling fluid connection and said electromagnetic wave energy propagating structure, means for coupling said helical section to said transmission line pedance to said transmission line section, said shorting plane being substantially perpendicular to the axis of said electromagnetic wave energy ropagating structure.

2. A microwave tube as is set forth in claim 1 wherein said transmission line section. comprises a first planar conductor means connected between said propagating structure and said transmission line coupling connection, and a second planar conductor means including a ground plane conductor positioned next to a part of said first planar conductor means to form a strip transmission line.

3. A microwave tube as is set forth in claim 2 wherein said ground plane conductor lies in a plane that is substantially perpendicular to said shorting plane, and means for connecting said helical section to said first conductor means.

4. A microwave tube as is set forth in claim 2 wherein said ground plane conductor lies in a plane that is substantially parallel with said shorting plane, and means for connecting the other end of said helical section to said first conductor means.

5. A microwave tube as is set forth in claim 1 wherein said transmission line section comprises a first conductor that extends between said transmission line coupling connection and said helical section for transferring electromagnetic energy between said transmission line cou-' pling connection and said helical section, and a second conductor that extends between said helical section and said electromagnetic energy propagating structure for transferring energy between said helical section and said propagating structure.

6. A microwave tube as is set forth in claim 5, further including a further shorting plane at the other end of said helical section.

7. A traveling wave tube comprising a vacuum envelope, a slow wave propagating structure positioned within said envelope, a transmission line coupling connection positioned externally of said envelope, a cooling fluid connection positioned externally of said envelope,

means positioned within said envelope for transferring electromagnetic energy between said coupling connection 8 and said slow wave propagating structure,'said meansfor transferring electromagnetic energy including a section of strip transmission line, means including a helical section for transferring acooling fluid between said cooling fluid connection and said slow wave propagating structure, means for connecting one end of said helical section to one side of said strip transmission line and means for positioning a shorting plane at an end of said helicalsection to form a resonant section at a center operating frequency for said traveling wave tube whereby said resonant section presents a high shunt impedance to said strip transmission line. v i

8. A traveling wave tube'cornprising a vacuum envelope, a slow wave propagating structure positioned within said envelope, 'a transmission line coupling connection positioned externally of said envelope, a cooling fluid connection positioned externally of said envelope, means positioned within said envelope for transferring electromagnetic energy between said coupling connec tion and said slow wave propagating structure, means including a helical section for transferring a cooling fluid between said cooling fluid connection and said slow propagating structure, means for shorting both ends of said helical section to form a resonant section that is an integral number of half wavelengths long at the center operating frequency for said traveling wave tube.

References Cited by the Examiner UNITED STATES PATENTS 2,800,605 7/1957 Marchese 31332 X 2,833,955 5/1958 Marchese 313-32 X 2,846,613 8/1958 Pierce 3153'.6 2,863,093 12/1958 Arditi 315-3.5 X 2,891,190 6/1959 Cohn 313-35 3,050,656 8/1962 Mayer 3153.5

, FOREIGN PATENTS 1,115,097 12/1955 France.

HERMAN KARL SAALBACH, PrimaryE x uminer.

P. L. GENSLER, Assistant Examiner. 

1. A MICROWAVE TUBE COMPRISING A VACUUM ENVELOPE, AN ELECTROMAGNETIC WAVE ENERGY PROPAGATING STRUCTURE POSITIONED WITHIN SAID ENVELOPE, A TRANSMISSION LINE COUPLING CONNECTION POSITIONED EXTERNALLY OF SAID ENVELOPE, AN ELECTROMAGNETIC ENERGY TRANSMISSION LINE SECTION POSITIONED WITHIN SAID ENVELOPE FOR TRANSFERRING ELECTROMAGNETIC ENERGY BETWEEN SAID COUPLING CONNECTION AND SAID PROPAGATING STRUCTURE, A COOLING FLUID CONNECTION POSITIONED EXTERNALLY OF SAID ENVELOPE, A HELICAL SECTION POSITIONED WITHIN SAID ENVELOPE FOR TRANSFERRING A COOLING FLUID BETWEEN SAID COOLING FLUID CONNECTION AND SAID ELECTROMAGNETIC WAVE ENERGY PROPAGATING STRUCTURE, MEANS FOR COUPLING SAID HELICAL SECTION TO SAID TRANSMISSION LINE SECTION, AND A SHORTING PLANE POSITIONED AT ONE END OF SAID HELICAL SECTION ADJACENT SAID COOLING FLUID CONNECTION TO FORM A RESONANT SECTION THAT PRESENTS A HIGH SHUNT IMPENDANCE TO SAID TRANSMISSION LINE SECTION, SAID SHORTING PLANE BEING SUBSTANTIALLY PERPENDICULAR TO THE AXIS OF SAID ELECTROMAGNETIC WAVE ENERGY PROPAGATING STRUCTURE. 